Lluís Vilar

Professor de la Universitat de Girona (UdG)

1. Why fires occur.

2. Why plants burn.

3. The causes of greater or lesser impact of fire.

4. Effects of fire on the flora

5. Effects of fire on the dynamics of the vegetation.

The effects of a fire are seen most clearly on the flora and vegetation of an area because of the profound transformation fire brings to the countryside. In this discussion we shall talk basically about the effects of fire on Mediterranean countryside.

Fire is a process of rapid combustion in which great quantities of energy are released which derive from the chemical energy of the vegetal biomass. There have been fires ever since there has been vegetation on Earth, that is, since the Devonian age some 400 million years ago.

Fire has been present throughout terrestrial plant evolution, as is indicated by the strategy used by the plants known as pyrofytes, which need fire in order to reproduce successfully.

Fire, then, is a natural phenomenon, has always been present in all terrestrial ecosystems and more frequent in drier environments. What humans have done is to increase that frequency.

Vegetation burns because it is made up of cellulose, lignin, hemi-cellulose and volatile substances, all of which are compounds that are susceptible to burning easily.

Fire begins with endothermic reactions which absorb energy ==> PRE-IGNITION

and continues with exothermic reactions which release energy. ==> COMBUSTION

In the warming up phase, (pre-ignition), water and volatile substances evaporate and so the heat is employed in the evaporation. The progression of the fire depends on:

1. the humidity of the combustible material
2. the water content of the material

These two factors explain why fires have greater or lesser impact.

Fires are closely linked to:

1) certain environmental conditions

==> periods of drought

==> dry winds

==> local humidity

2) the conditions combustible material (whether it is more or less humid depends on 1) and its water content, HC.

On the whole fires have negative effects arising from the modification of the habitat and all the organisms living there are affected negatively. However, recuperation is quick and in a few years the area returns to the initial state it was in before the fire. After a fire certain factors have to be considered:

1. loss of nutrients
2. loss of protection from the sun
3. loss of capacity to retain and infiltrate water
4. partial or total combustion that the vegetation has been subject to.

There are two main recuperation strategies after a fire:

1. sprouting plants
2. germinating plants

The prevalence of one or the other of these strategies depends on the frequency of fires-->

More frequent fires ==> more germinating species

Less frequent fires ==> more sprouting species.

The islands of vegetation that have not burnt help to colonise the affected area as it is a place which provides offspring of the organisms that remain, and this offspring can colonise the free space left behind after the fire.

As well as fire, the flora also suffers the effects of animal and human activity and it is the combination of these effects that explain the different environments, the different landscapes and the different flora of an area. Some species are so adapted to a particular environment that they can only prosper if this environment is maintained.

To understand the effects of fire on the vegetation and the landscape of an area, it is important to consider not only the frequency of fire, but also three other important factors:

1. wind
2. rainfall
3. human activity

The combination of these four factors -acting together, not separately - is what explains the effects of fire on a place. The more fire, wind and human activity there is, the less forest there is and vice versa. The frequency of fires, therefore, does not in itself account for the vegetation growing in a certain area: wind, rainfall and above all, human activity are the other factors that must be studied to understand the reasons for the different types of plants there are in a particular place.

In short, in the Mediterranean lands, fire is a disturbance which moulds the countryside and creates diversity, but when it is very frequent, it can bring uniformity of the vegetation and reduce the biodiversity of an area.